Background on Intubation Devices:
Intubation devices provide passage to gases, to mechanically ventilate the lungs, and to apply anesthetic agents and certain medications. Typically, they are connected through a catheter mount to machines that pump and control the flow of such gases (herein, the term “Ventilator” refers to such machines).
Intubation devices include endotracheal tubes, tracheostomy tubes, laryngeal mask airways, endobronchial tubes, and supraglottic tubes. In this specification, the term “Tube” refers to the conduit that such devices use for the ventilation gases, while the term “Duct” refers to significantly smaller diameter conduits, which are typically used for purposes such as inflating cuffs, and suctioning fluids.
To control the type and amount of substances flowing into a patient's lungs, it is necessary to seal the space around the tube, so that gases are forced to flow in and out of the lungs exclusively through the tube's ventilation lumen. In addition, the seal prevents contaminated fluids from entering the lungs. In this specification, the term “Contaminated Fluids” (CFs) refers to fluids such as nasopharyngeal secretions, esophageal reflux, blood, and debris.
Endotracheal Tubes (ETTs) usually have a seal consisting of an inflatable cuff (herein, “Tracheal Cuff”) that surrounds the distal portion of the tube. After an ETT is placed in the trachea, the cuff is inflated, so it fills the tracheal passage around the tube, and thus provides an annular seal. Cuffs are soft and collapsible, to allow insertion of the device without damaging the vocal chords, or the tracheal mucosa. The cuffs are inflated through a small duct, which connects to a small-diameter lumen inside of the tube's wall. This lumen, in turn, connects to the cuff through an opening in the tube's wall.
Laryngeal Mask Airways (LMAs) do dot provide as good a seal as ETTs, but are easier to place. LMAs have an inflatable seal that consists of an oval mask, with an attached peripheral ring. When the peripheral ring is inflated, it causes the mask to form a seal against the glottis. Unlike ETTs, LMAs do not risk damaging the vocal chords, or the trachea, as their tubes does not enter the trachea. Instead, LMAs have a tube that terminates at an opening of the mask. This opening allows the passage of ventilation gases, while the mask confines them to the tube. Herein, “Inflating the Seal”, when referring to LMAs, means inflating the corresponding ring.
Blind-Insertion Airways (BIAs) have an oropharyngeal cuff, and a smaller esophageal cuff. Gases flow to the lungs through a lateral opening in the tube, located between the oropharyngeal cuff and the esophageal cuff. BIAs with a dual lumen can provide a patent airway even if the esophageal cuff is accidentally placed in the trachea. BIAs are easier to place than ETTs, but do not provide as good a seal. Also, certain medical conditions, or injuries, may prevent using BIAs.
Different devices have been developed to guide the insertion and proper placement of intubation devices. For example: (a) markings on the tube wall can be aligned with specific anatomic features, such as the vocal chords, (b) radiographically opaque lines have been attached, to visualize and control the proper insertion depth, and (c) imaging devices facilitate viewing the vocal chords and other anatomical features as the tubes are placed.
Background Related to Leakage:
To avoid accumulation of CFs on the proximal side of the cuff, suctioning devices have been incorporated to ETTs. Suctioning lines connect proximally to a vacuum source, and distally to a small hole in the ETT tube, which is located by the cuff's proximal side. However, under some circumstances, the suctioning holes may be blocked. Consequently, CFs may accumulate. To improve the suctioning, in some devices, the suctioning hole connects to a manifold wrapped flat against the ETT wall, and has multiple suctioning entrances. However, given the geometry of the cuffs and the trachea, not all CFs can be removed. Also, depending on the viscosity of the CFs, suction holes and manifolds can get fully or partially obstructed. In such situations, some CFs can still accumulate, and eventually leak past the cuff.
Given variations in tracheal sizes, cuffs may not be able to expand fully, causing folds in the cuff portions in contact with the trachea. Even micro-channels created by such folds can serve as leak paths that allow CFs to flow past the cuff and eventually reach the lungs. Various sizes, shapes, and materials are used to reduce such leaks. For example, some cuffs are tapered, instead of cylindrical. Cuffs are typically made out of PVC, silicone, or polyurethane. Thinner and more compliant cuffs, typically made out of polyurethane, have been shown to reduce (but not eliminate), leakage of CFs past the cuff. Moreover, thinner cuffs are more delicate, so the risk of rupture is higher.
Background Related to Gas Pressures:
Typically, cuffs are inflated manually. A one-way valve keeps the gases inside the cuff, and the pressure is monitored with a pilot balloon, or with a manometer. Herein, the term “Pressurizer” means a device (or other source) that supplies gas, at a flow rate and pressure appropriate to inflate cuffs, inflate LMA rings, inflate balloons, and in general keep enclosed cavities at a specified pressure. Herein, the terms “Manual Pressurizer”, or “Inflator”, mean pressurizers consisting of a syringe pump, or equivalent, and operated manually. Herein, the term “Cuff Pressurizer” refers to pressurizers used to inflate cuffs.
Medical practitioners can control and monitor various parameters related to the gases that flow in and out of the lungs of the intubated patient, including the composition of the gases, the flow rate, and various pressures. Two such pressures are the Positive Inspiratory Pressure (PIP), and the Positive End Expiratory Pressure (PEEP).
As the ventilator runs through a breathing cycle, the pressure of the gases on the distal side of the cuff varies between a maximum of PIP and a minimum of PEEP. Since a higher pressure on the distal side of the cuff helps to limit the leakage of CFs past the cuff, the risk of leakage is highest at PEEP.
Herein, all numerical pressure values are expressed in cm of H2O, with 0 cm H2O corresponding to the atmospheric pressure. It is common to set PEEP slightly above atmospheric pressure (e.g., 4-5 cm H2O), to reduce the risk of alveolar damage or collapse. Hypoxemia, and other medical conditions may require higher PEEP values. However, if PEEP is too high, it can cause reduced venous blood flow, undesirable changes in blood chemistry, and other medically unacceptable side effects. Hence, these and other medical reasons limit both the admissible lower and upper values of PEEP, and determine their optimal values for each patient and pathology.
In general, higher cuff pressures reduce capillary flow in the tracheal mucosa. With a pressure of under 20 cm H2O the tracheal mucosa is normally perfused, but at 30 cm H2O it is already slightly blanched, and at 40 cm H2O arterioles are occluded. Excessive cuff pressure, depending on its duration and magnitude, can cause damage to the tracheal tissue, including ischemia, necrosis, and ulceration. Cuff inflation typically targets pressures of approximately 20 to 30 cm H2O, as a suitable compromise between the impact of the cuff on the tracheal mucosa, and how well it seals the space surrounding the tube.
Seepage of CFs between the cuff and the trachea could be avoided altogether if there were no constraints on how much PEEP and the cuff pressure can be raised. However, these pressures are constrained, because of the reasons explained above. Therefore, simply raising PEEP or the cuff pressure to obtain an assured seal, are not viable options.
Unmet Needs Related to Intubation:
In conclusion, in spite of proper use of medical protocols and techniques, and of the aforementioned improvements to intubation devices, small amounts of CFs still can leak past the cuffs. Furthermore, some of these CFs can get aerosolized, and thus carry bacteria deeply into the lungs. Serious infections associated with the use of intubation are common, particularly with longer intubation times. Such infections frequently lead to Ventilator Associated Pneumonia (VAP), and increase morbidity and mortality rates. Thus, there remains a considerable need for improved intubation devices and methods.